Method for fabricating single-mode DBR laser with improved yield

ABSTRACT

A method for fabricating a laser for generating single-longitudinal mode laser light at a lasing wavelength. A semiconductor active region for amplifying, by stimulated emission, light in the laser cavity at the lasing wavelength is formed. A grating is formed adjacent to the active region, the grating having a grating period corresponding to a Bragg wavelength substantially equal to the lasing wavelength. An intermediate section of the grating is removed to result in first and second pluralities of gratings separated by a gratingless intermediate section. First and second grating sections are formed comprising the first and second pluralities of gratings, where the first and second grating sections each have a first effective index of refraction. A gratingless phase-shift section is formed in said intermediate section, the phase-shift section being disposed adjacent to the active region and between the first and second grating sections and having a second index of refraction different than the first index of refraction. The phase-shift section has a length sufficient to impart a phase shift for light at the lasing wavelength sufficient to achieve single-longitudinal mode operation. Alternatively, a grating material layer is deposited adjacent to the active region, the grating material layer having a first section, a second section, and an intermediate section between the first and second sections. Gratings are formed in the first and second sections to form a first grating section in the first section, a second grating section in the second section, leaving a gratingless phase-shift section in the intermediate section.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending application Ser. No.10/159,347 filed on May 31, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to distributed Bragg reflector (DBR) lasers and,in particular, to DBR lasers having a quarter-wavelength (λ/4) phaseshift section for improved single-longitudinal mode operation thereof.

2. Description of the Related Art

The following descriptions and examples are not admitted to be prior artby virtue of their inclusion within this section.

There are several types of lasers, including gas lasers, solid-statelasers, liquid (dye) lasers, free electron, and semiconductor lasers.All lasers have a laser cavity defined by at least two laser cavitymirrors, and an optical gain medium in the laser cavity. The gain mediumamplifies electromagnetic waves (light) in the cavity by stimulatedemission, thereby providing optical gain.

In semiconductor lasers, a semiconductor active region serves as thegain medium. Semiconductor lasers may be diode (bipolar) lasers ornon-diode, unipolar lasers such as quantum cascade (QC) lasers.Semiconductor lasers are used for a variety of industrial and scientificapplications and can be built with a variety of structures andsemiconductor materials.

The use of semiconductor lasers for forming a source of optical energyis attractive for a number of reasons. Semiconductor lasers have arelatively small volume and consume a small amount of power as comparedto conventional laser devices. Further, semiconductor lasers can befabricated as monolithic devices, which do not require a combination ofa resonant cavity with external mirrors and other structures to generatea coherent output laser beam.

The optical gain of a laser is a measure of how well a gain medium suchas an active region amplifies photons by stimulated emission. Theprimary function of the active region in a semiconductor laser is toprovide sufficient laser gain to permit lasing to occur. The activeregion may employ various materials and structures to provide a suitablecollection of atoms or molecules capable of undergoing stimulatedemission at a given lasing wavelength, so as to amplify light at thiswavelength. The active region may comprise, for example, a superlatticestructure, or a single- or multiple quantum well (MQW) structure.

Amplification by stimulated emission in the active region of asemiconductor laser is described as follows. The semiconductor activeregion contains some electrons at a higher, excited state or energylevel, and some at a lower, resting (ground) state or energy level. Thenumber and percentage of excited electrons can be increased by pumpingthe active region with a pumping energy, from some energy source such asan electrical current or optical pump. Excited electrons spontaneouslyfall to a lower state, “recombining” with a hole. The recombination maybe either radiative or non-radiative. When radiative recombinationoccurs, a photon is emitted with the same energy as the difference inenergy between the hole and electron energy states.

Stimulated emission, as opposed to spontaneous emission, occurs whenradiative recombination of an electron-hole pair is stimulated byinteraction with a photon. In particular, stimulated emission occurswhen a photon with an energy equal to the difference between anelectron's energy and a lower energy interacts with the electron. Inthis case, the photon stimulates the electron to fall into the lowerenergy state, thereby emitting a second photon. The second photon hasthe unique property that it has the same energy, frequency, and phase asthe original photon. Thus, when the photons produced by spontaneous (orstimulated) emission interact with other high energy state electrons,stimulated emission can occur so that two photons with identicalcharacteristics are present. (Viewed as waves, the atom emits a wavehaving twice the amplitude as that of the original photon interactingwith the atom.) I.e., one photon of a given. energy, frequency, andphase produces a second photon of the same energy, frequency, and phase;and these two photons may each, if not absorbed, stimulate furtherphoton emissions, some of which can themselves stimulate furtheremissions, and so on.

Amplification by stimulated emission requires that more photons areproduced by stimulated emission than are absorbed by lower-stateelectrons. This condition, known as population inversion, occurs whenthere are more excited (upper lasing level) electrons than ground-state(lower lasing level) electrons. If there were more lower state thanupper state electrons, then more photons would be absorbed by the lowerenergy electrons (causing upward excitations) than would be produced bystimulated emission. When there is a population inversion, however,enough electrons are in the excited state so as to prevent absorption byground-state electrons from sabotaging the amplification process. Thus,when population inversion is achieved, stimulated emission predominatesover stimulated absorption, thus producing amplication of light (opticalgain). If there is population inversion, lasing is therefore possible,if other necessary conditions are also present.

Population inversion is achieved by applying a sufficient pumping energyto the active region, to raise a sufficient number of electrons to theexcited state. Various forms of pumping energy may be utilized to exciteelectrons in the active region and to achieve population inversion andlasing. For example, semiconductor lasers of various types may beelectrically pumped (EP), by a DC or alternating current. Opticalpumping (OP) or other pumping methods, such as electron beam pumping,may also be used. EP semiconductor lasers are typically powered byapplying an electrical potential difference across the active region,which causes a current to flow therein. As a result of the potentialapplied, charge carriers (electrons and holes) are injected fromopposite directions into an active region. This gives rise to anincrease in spontaneous generation of photons, and also increases thenumber of excited state electrons so as to achieve population inversion.

In a semiconductor laser, an active region is sandwiched between thecavity mirrors, and pumped with a pumping energy to cause populationinversion. Photons are spontaneously emitted in the active region. Someof those photons travel in a direction perpendicular to the reflectorsof the laser cavity. As a result of the ensuing reflections, the photonstravel through the active region multiple times, being amplified bystimulated emission on each pass through the active region. Thus,photons reflecting in the cavity experience gain when they pass throughthe active region. However, loss is also experienced in the cavity, forexample by extraction of the output laser beam, which can be about 1% ofthe coherent cavity light, by absorption or scattering caused by lessthan perfect (100%) reflectance (reflectivity) of the cavity mirrors,and other causes of loss.

Therefore, for lasing to occur, there must be not only gain(amplification by stimulated emission) in the active region, but enoughgain to overcome all losses in the laser cavity as well as allow anoutput beam to be extracted, while still allowing laser action tocontinue. Gain is a function of wavelength. The minimum gain that willpermit lasing, given the cavity losses, for a given wavelength orwavelength range, is the threshold lasing gain of the laser medium forthat wavelength or range. A given wavelength is associated with a giventhreshold gain, and may be characterized by that threshold gain, for agiven laser structure. (For EP lasers, the lowest drive current level atwhich the output of the laser results primarily from stimulated emissionrather than spontaneous emission is referred to as the lasing thresholdcurrent.)

When the active region provides the threshold lasing gain over a givenwavelength range, there will be a sufficient amount of radiativerecombinations stimulated by photons, so that the number of photonstraveling between the reflectors tends to increase, giving rise toamplification of light and lasing. This causes coherent light to buildup in the resonant cavity formed by the two mirrors, a portion of whichpasses through one of the mirrors (the “exit” mirror) as the outputlaser beam.

Because a coherent beam makes multiple passes through the opticalcavity, an interference-induced longitudinal mode structure or wave isobserved. The wave along the laser cavity is a standing EM wave and thecavity of effective optical length L only resonates when the effectiveoptical path difference between the reflected wavefronts is an integralnumber of whole wavelengths (the effective cavity length or optical pathdifference takes phase-shifting effects at the mirrors into account). Inother words, lasing is only possible at wavelengths for which theround-trip phase is a multiple of 2π. The set of possible wavelengthsthat satisfy the standing wave condition is termed the set oflongitudinal modes of the cavity. Although there are an infinite numberof such wavelengths, only a finite number of these fall within thewavelength range over which the gain spectrum of the active regionexceeds the threshold lasing gain. The laser will lase only at one ormore of the possible longitudinal (wavelength) modes which fit into thiswavelength range.

Semiconductor lasers may be edge-emitting lasers or surface-emittinglasers (SELs). Edge-emitting semiconductor lasers output their radiationparallel to the wafer surface, in contrast to SELs, in which theradiation output is perpendicular to the wafer surface, as the nameimplies. In conventional Fabry-Perot (FP) edge-emitting lasers, acleaved facet mirror is used to obtain the feedback for laseroscillation. In an FP laser, the two facets of the diode, the rear andthe front (emitting) surface, are cleaved to establish the dimensions ofthe structure such that a primary longitudinal mode of resonance willexist at the desired wavelength.

Other semiconductor lasers, such as distributed-feedback (DFB) anddistributed-Bragg reflector (DBR) lasers, employ one or more diffractiongratings to provide reflectance. The diffraction grating, also known asa Bragg grating, contains grating rows, stripes or “teeth” arranged in aregular pitch for distributively feeding light back by Bragg reflection.Such devices provide feedback for lasing as a result of backward Braggscattering from periodic variations of refractive index, instead ofusing conventional facets or mirrors to provide reflection.

A DFB laser typically has a uniform diffraction grating constructed inthe waveguide itself, adjacent to (i.e., above or below) the activeregion layer. In such a laser, optical feedback is provided along thelaser's cavity length by means of a diffraction grating whose stripes orteeth run perpendicular to the length (longitudinal direction) of thelaser cavity. The grating serves as an internal periodic feedbackstructure that establishes the wavelength of operation.

The term “DBR laser” typically denotes an edge-emitting laser comprisinga grating that is not uniform, i.e. a grating that is not continuousover the entire longitudinal cavity. A typical is DBR laser has one ortwo grating sections fabricated in a waveguide external to the activeregion, which grating sections provide part or all of the reflectance ofthe cavity mirrors. For example, the DBR laser typically is anedge-emitting laser having two separate DBRs or gratings, each at eitherend of, and outside, the active region layer. The DBRs reflect and thusprovide feedback at the desired wavelength.

Diffraction gratings provide optical feedback by the period variation ofthe effective refractive index of the grating. The period indexvariation causes a wavelength-selective feedback. Thus, the diffractiongratings of DFB and DBR lasers have a reflectance that is a function ofwavelength. Maximum reflectance of a diffraction grating occurs aroundits so-called Bragg wavelength (λ_(B)), which is given by Eq. (1) below:

λ_(B)=2Λn _(e)  (1)

where Λ is the period (pitch) of the grating and n_(e) is the effectiverefractive index of the waveguide having the grating. Thus, the Braggwavelength λ_(B) is determined by the grating pitch or period. Theperiod of DFB and DBR gratings is typically selected so that the Braggwavelength λ_(B) is equal to the desired operating wavelength of thelaser. Diffraction gratings therefore give significant reflection, andthus significant optical feedback, only around the operating wavelength.

Single-wavelength (single longitudinal mode) operation is desirable formany applications, such as in high-bit-rate optical fiber communication.Single-mode operation refers to laser operation in which the intensityof the most-intense, or “primary,” mode lasing is substantially greaterthan all other modes, including the next-most-intense and adjacentmodes. Single-mode operation may be said to occur when the side modesuppression is at least a minimum amount (e.g., 30 dB), for example, theprimary mode is at least 30 dB greater than its side modes (and allother modes). In telecommunications applications, for example, it isdesirable that the laser emit at a single lasing wavelength at 1.31 μm(and other closely spaced wavelengths), or at telecommunicationswavelengths specified by the ITU grid, such as lasing wavelengths of1.55 μm (and other closely spaced wavelengths). These wavelength rangesare often used for telecommunications purposes because the loss ofsilica fibers is comparatively low at these wavelengths.

In a conventional DFB laser, an additional phase shift π is introducedat the Bragg wavelength λ_(B). Therefore, the round-trip phase at theBragg wavelength λ_(B) is π+n2π, where n is an integer. However, π+n2πis not a multiple of 2π. Thus, the phase condition for oscillationcannot be met at the Bragg wavelength λ_(B) (or corresponding Braggfrequency f_(B)), and no mode occurs at this wavelength. Instead, thereare two equally “primary” modes, at two frequencies slightly removed bysome frequency f from the Bragg frequency f_(B), i.e. two optical modeshaving frequencies f_(B)±f are present. This means that there are twomodes having approximately equal threshold gain. These two modes aresometimes referred to as the right Bragg mode and the left Bragg mode.

The presence of these two “co-primary” modes can give rise to anunpredictable lasing frequency, since oscillation often occurs,unpredictably, in either one of the modes, as these two so-calleddegenerate modes compete substantially equally for lasing as thedominant mode. Mode-hopping can also occur from one mode to the other.The two modes are thus problematic for wideband applications, since asingle, narrow linewidth output is needed for wideband applications. Twomodes having similar threshold gain are sometimes known as degeneratemodes. The existence of two degenerate modes results in multi-modeoperation, unpredictable modes, or mode hopping, sometimes referred toas mode degeneracy.

It is preferable to construct a laser in which there is only a singleprimary mode, i.e. in which there is no mode degeneracy. The insertionof a π/2 optical phase shift section into the DFB structure is one wayto “suppress” or “break” the mode degeneracy, so as to achievesingle-mode operation. This phase shift region creates an extra π/2phase shift for each wave passing along it of wavelength λ_(B). The π/2phase shift section thus provides an extra round-trip phase shift of π,which adds to the additional phase shift n introduced by a conventionalDFB laser at the Bragg wavelength λ_(B), so that the round-trip phase isa multiple of 2π at the Bragg wavelength λ_(B). That is,π+(π+n2π)=2π+n2π=(n+1)2π, which is an integer multiple of 2π, since(n+1) is an integer.

Thus, with a π/2 phase shift section in the grating, the phase conditionfor oscillation can be met at the Bragg wavelength λ_(B) (or Braggfrequency f_(B)). In effect, the π/2 phase shift technique provides foroptical phase matching to adjust the main mode at the Bragg wavelength.This “reduces” the number of resonance modes to one, and its resonancefrequency coincides with the Bragg frequency f_(B). Thus, the π/2 phaseshift section effectively provides lasing predominantly on a preferredand fixed Bragg mode. The result is a laser that has only one primarymode, i.e. only one longitudinal mode with the lowest threshold gain atthe Bragg wavelength, thereby achieving stable single mode oscillationat the Bragg wavelength. (Conventional DBR lasers typically employ anactively controlled phase-shift section in which current is injected tocontrol the phase shift.)

The π/2 phase shift of this technique is also known as a quarter-lambda,quarter-wavelength, or λ/4 phase shift. This is because introducing aπ/2 phase shift at λ_(B) is equivalent to adding a section of lengthΛ/2=λ_(B)/(4n_(e)) into the grating structure. Although λ₄ is literallya length, not a phase shift, the terminology “λ/4 phase shift” will beemployed herein due to the conventional use of this term. It will beunderstood that a “λ/4 phase shift”, as the term is used in thisapplication, refers to a phase shift of π/2, at wavelength λ_(B), whichphase shift is equivalent to the phase shift that would be obtained if asection of length Λ/2 =λ_(B)/(4n_(e)) were to be inserted into theoptical path, i.e. the phase shift (at λ_(B)) resulting from increasingthe optical path by length λ_(B)/4.

Various approaches have been employed to shift the optical phase by π/2in DFB lasers. For example, to achieve a π/2 shift, many commerciallasers simply insert lithographically a physical λ/4 phase shift intothe grating mask. The resultant devices are known as λ/4 or π/2phase-shifted DFB lasers and oscillate with a single frequency close tothe Bragg frequency of the grating. Such structures employing directlyphase-shifted gratings are described in “Stability in SingleLongitudinal Mode Operation in GaInAsP/InP Phase-Adjusted DFB Lasers,”by Haruhisa Soda et al., IEEE J. Quantum Electronics, vol. QE-23, No. 6,June 1987, pp. 804-814 (Haruhisa Soda Reference), “Asymmetricλ/4-Shifted InGaAsP/InP DFB Lasers,” Masashi Usami et al., IEEE J.Quantum Electronics, vol. QE-23, No. 6, June 1987, pp. 815-821 (MasashiUsami Reference); Distributed Feedback Semiconductor Lasers, by JohnCarroll, James Whiteway & Dick Plumb (London: Institution of ElectricalEngineers, 1998) (Carroll Reference), section 1.7.2 (pp. 26-28); andHandbook of Distributed Feedback Laser Diodes, by Geert Morthier &Patrick Vankwikelberge (Boston: Artech House, Inc., 1997), section4.1.4, pages 102-104 (Morthier & Vankwikelberge Reference).

Other techniques for shifting the optical phase by π/2 include:employing a nonuniform waveguide structure, as described in the HaruhisaSoda Reference; providing a waveguide having two straight portions and abending portion, so that the longer bending portion causes aquarter-lambda shift, as described in U.S. Pat. No. 4,833,687; changingthe thickness of the active layer in a phase shift section, as describedin U.S. Pat. No. 4,847,856; and moving the left and right sections ofthe grating with respect to each other in a direction perpendicular tothe longitudinal axis of the active region, as described in U.S. Pat.No. 5,052,015.

However, there are various disadvantages with conventional λ/4phase-shifting techniques. For example, directly inserting a physicalλ/4 phase shift into the grating mask can be difficult to manufacture,because it requires multiple grating fabrication steps, for example.Also, such phase-shift sections inserted directly into the corrugationof the grating does not give optimum dynamic wavelength stability duringmodulation, requiring more complex phase-adjusting techniques such asinsertion of two λ/8 phase shift sections, which can add to thedifficulties of manufacture.

Further details of DFB lasers and diffraction gratings may be found inthe Carroll Reference and in the Morthier & Vankwikelberge Reference.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent uponstudy of the following description, taken in conjunction with theattached FIGS. 1-4.

FIG. 1 is a cross-sectional view of the layer structure of a DBR laserhaving a phase-shift section, in accordance with an embodiment of thepresent invention;

FIG. 2 is a cross-sectional view of an alternative phase-shift sectionand grating layer, in accordance with an embodiment of the presentinvention;

FIGS. 3A-H are cross-sectional views of the layer structure of the DBRlaser of FIG. 1 at various stages of fabrication, in accordance with anembodiment of the present invention; and

FIGS. 4A-E are cross-sectional views of the layer structure of a DBRlaser employing the alternative phase-shift section of FIG. 2 at variousstages of fabrication, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a single-mode DBR laser with an improvedphase-shift section and a method for fabricating a single-mode DBR laserwith improved yield. The present invention comprises an edge-emittingsemiconductor DBR laser having two DBR gratings adjacent to oppositeends of the active region layer, and a quarter-wavelength phaseshift-section. disposed longitudinally between the two DBR gratingsections so that it is adjacent to an intermediate portion of the activeregion layer between its opposing ends. The phase shift section is asection where there is no grating, so that the phase shift section hasan effective index of refraction different from that of the two DBRgratings. The length of the grating section and the relative effectiveindexes of refraction of the phase-shift section and the gratingsections are selected to achieve a phase shift approximately equal toπ/2, in a preferred embodiment, or enough of a phase shift to suppressmode degeneracy so that there is only one primary mode.

The DBR laser of the present invention may be utilized for variousapplications such as telecommunications applications. For example, theDBR laser may be designed to emit at 1.31 μm or 1.55 μm wavelengths,where optical fibers have lower attenuation. Further details,advantages, and embodiments of the invention are described below withreference to FIGS. 1-4.

Referring now to FIG. 1, there is shown a cross-sectional view of thelayer structure of a DBR laser 100 having a gratingless (i.e.,grating-free) phase-shift section 142, in accordance with an embodimentof the present invention. DBR laser 100 is an edge-emitting laser havingbottom contact layer 111, substrate 112, buffer/cladding layer 113,bottom confinement layer 121, active region 120, top confinement layer122, spacer layer 145, grating layer 140, and top contact layer 133.Laser 100 has anti-reflectance (AR) coatings 101 and 102 on its facets.

Laser 100 comprises layers and structures. fabricated on substrate 112by epitaxial or other deposition techniques. In particular, theepitaxially grown layers of laser structure 100 may be fabricated on atop major surface of wafer substrate 112 with epitaxial growthtechniques such as molecular beam epitaxy (MBE), liquid phase epitaxy(LPE), a vapor phase epitaxy (VPE) process such as or metalorganicchemical vapor deposition (MOCVD, also known as MOVPE), or other knowncrystal growth processes. Other layers and structures that are notepitaxially grown may be deposited with non-epitaxial depositiontechniques such as e-beam evaporation, thermal evaporation, orsputtering.

Grating layer 140 has an upper cladding layer 132 (consisting ofmaterial 146), first (“exit,” or “front”) grating section 141, second(“back”) grating section 143, and a phase-shift section 142 sandwichedbetween the first and second gratings 141, 143. Gratings 141, 143 eachcomprise a plurality of diffraction grating stripes, rows or teeth 144that run in the x direction, perpendicular to the length (longitudinaldirection y) of the laser cavity.

First grating section 141 is approximately of length L1 and comprises agrating having a grating pitch or period selected to have a Braggwavelength λ_(B) equal to the desired operating wavelength of the laser.Second grating section 143 is approximately of length L3 and alsocomprises a grating having a grating pitch selected to have the sameBragg wavelength λ_(B). As will be appreciated, gratings 141, 143comprise a much larger number of grating periods than illustrated in thefigures, which are not drawn to scale for simplicity of illustration.For. example, each of grating sections 141, 143 covers hundreds ofgrating periods in various practical applications and embodiments.Phase-shift section 142 is a grating-free phase-shift section interposedbetween the first and second gratings, and is of approximately lengthL2, and also covers a length equal to hundreds of grating periods intypical embodiments. In an embodiment, the gratings of sections 141, 143are “in phase” with each other, and the gratingless phase-shift section142 covers an length between the first and second grating sections 141,143 which is “missing” grating periods that would otherwise be in phasewith the grating periods of grating sections 141, 143.

Phase-shift section 142 may be referred to as a “distributed”phase-shift section because it extends over a substantial number of(missing) grating periods, between the first and second gratingsections, and is of comparable length to these distributed gratingreflectors 141, 143. For example, in preferred embodiments, length L2 isno less than {fraction (1/10)} the length of the smaller of lengths L1,L3. In various practical embodiments, none of lengths L1, L2, and L3 ismore than two or three times the length of the others.

In an embodiment, bottom and top contact layers 111, 133 consistprimarily of a metal such as Au. Other materials may be employed inalternative embodiments. Top contact layer 133 may also comprise a thin(e.g., about 250 Å to 500 Å thick) contact-facilitating layer (notshown) between the metal (Au) and cladding 132, comprised of a materialsuch as doped InGaAs (with a doping density of about 5×10¹⁹/cm³), toprovide for electrical contact between the cladding 132 and the metal ofthe metal contact layer portion of contact 133. (e.g. a ground or otherelectrical terminal or wire bond). Probes or wire bonds may beelectrically coupled to top and bottom contacts 111, 133 to providepumping current to laser 100, so as to give rise to populationinversion.

In an embodiment, substrate 112, buffer/cladding layer 113, spacer layer145, and material 146 of grating layer 140 (including cladding layer132) consist of InP, which is often used as a substrate material for1.55 μm and 1.31 μm devices. In an embodiment, substrate 112 is about120 μm thick; buffer/cladding layer 113 is about 0.8 μm thick; spacerlayer 145 is about 40 nm thick; and cladding layer 132 is about 1.2 μmthick. Other thicknesses are possible in alternative embodiments.

Active region 120, in an embodiment, comprises a structure such asmultiple quantum wells (MQWS) which provide a gain spectrum suitable foremitting radiation at the desired wavelength, e.g. 1.31 μm. Activeregion 120 may consist of a material such as (undoped) InGaAsP/InP (orInAlGaAs/InP) having, for example, seven quantum wells (QWs) (each QWhaving a QW layer and a corresponding QW barrier layer). In alternativeembodiments, active region 120 could have a single quantum well,superlattice, or other structure instead of an MQW structure. In anembodiment, active region 120 is about 0.1 to 0.2 μm thick.

Quaternary compounds or alloys composed of elements from the third group(Al, Ga, In) and the fifth group (P, As, Sb) of the periodic system(sometimes called III-V semiconductor compounds) are often employed indevices using InP substrates because they may be lattice-matched to InP.Confinement layers 121, 122 have an index of refraction selected to begreater than that of the adjacent InP-based buffer/cladding layer 113and spacer layer 145, so as to confine more of the optical field to theactive region 120, which confinement layers 121, 122 surround.

The bandgap of a quaternary material, measured in eV, is typicallyspecified in terms of the wavelength of light at which photons have thesame energy as the bandgap. Thus, for example, a material having abandgap equal to the energy of photons of a wavelength of 1.1 μm may besaid to have a bandgap of 1.1 μm. In an embodiment, confinement layers121, 122 consist of a quaternary material, such as InGaAsP,lattice-matched to InP and having a bandgap of about 1.1 μm, so as toprovide an index of refraction of about 3.3, which is greater than theindex of refraction of the adjacent InP layers (i.e., 3.2). Confinementlayers 121, 122 are about 100 nm thick in an embodiment.

AR coatings 101, 102 consist of a Ta₂O₅/SiO₂ mixture and are about 1 μmthick, in an embodiment. Such AR coatings provide, in an embodiment,about 0.1% reflectance.

The diffraction grating stripes 144 consist of a material having anindex of refraction n₁ which is different from the index of refractionn₂ of the surrounding material 146 of cladding layer 132 and spacerlayer 145. In an embodiment, n₁>n₂. For example, where material 146 isInP, n₂=3.2. The grating material from which grating stripes 144 aremade may consist of quaternary material lattice-matched to InP, such asInGaAsP, and having a bandgap selected to provide a desired (e.g.,optimal) index of refraction n₁. For example, the relative moleconcentrations for the InGaAsP compound may be selected to achieve abandgap of about 1.18 1.1 μm, so that n₁=3.4.

Depending on the cross-section, size, pitch, and duty cycle of gratingsections 141, 143, and the values of n, and n₂, as well as the indexesof refraction of other layers “seen” by light in the cavity, gratingsections 141, 143 will have an effective index of refraction n_(G)(where “G” stands for “grating”). In an embodiment, with values of n₁and n₂ as given above, and other details for the material andthicknesses of other layers of laser 100, n_(G) is about 3.2333.Similarly, the effective index of refraction of phase-shift section 142,n_(PS), is about 3.23 (where “PS” stands for “phase-shift”). Index N_(G)is close to, but slightly greater than, n_(PS).

The presence of higher index of refraction grating stripes 144 withingrating sections 141, 143 (i.e., n₁>n₂) makes the effective index ofrefraction N_(G) of grating sections 141, 143 slightly greater than thatof phase-shift section 142, which does not contain these higher-indexstripes. That is, the presence of a grating section having alternatingindex of refraction sections ensures that N_(G) will be different thann_(PS), which does not have a grating. Thus, by virtue of phase-shiftsection 142 being gratingless, it has an index of refraction n_(PS)necessarily different than the index of refraction N_(G) of an adjacentgrating section. However, in practical embodiments, such as InP-baseddevices for applications such as 1.31 μm or 1.55 μm emission, n_(G),although slightly greater than n_(PS), will be very close to n_(PS).This is because index n₁ of grating stripes 144 is 3.4, close to 3.2,the index of refraction n₂ of the surrounding material 146 of claddinglayer 132. Thus, by adding a comparatively small amount of material ofindex 3.4 into material of index 3.2, the effective index changes, butonly by a small amount, i.e. from about 3.23 to about 3.2333 in theillustrated embodiments.

The lengths LI, L3 of grating sections 141, 143, in an embodiment, aredifferent (L1<L3), to provide an asymmetric reflectance. This causesmore laser light to be output through the lower reflectance mirror 141,so that the laser puts out two laser beams: a primary, exit beam 151(exiting through the side with shorter, lower-reflectance gratingsection 141 and AR coating 101) and a weaker beam 152 (exiting throughthe side having higher-reflectance, longer grating section 153 and ARcoating 102). The stronger, primary beam 151 may be coupled to someexternal light-receiving device such as a lens, fiber, amplifier,modulator, and so on. The weaker beam 152 may be directed onto a monitorphotodiode (not shown) for power monitoring or other purposes.

Typically, a given power ratio (the ratio of the power of the primaryoutput beam 151 to the power of the secondary beam 152), such as 4:1 orsome other ratio, may be desired. The greater the back-to-frontreflectance ratio of the cavity mirrors, the greater the power ratio,although the relationship is not linear. Similarly, the reflectance of agrating section increases with length, but not in a linear fashion.Therefore, the back-to-front reflectance ratio is a non-linear functionof the back-to-front grating section lengths L3, L1 and their ratioL3:L1. Thus, for a given grating and waveguide structure, gratingsection lengths L1 and L3 are selected, as described below, to achieve adesired reflectance ratio (e.g., about 2:1 or 3:1), so that output beam151 is stronger than secondary beam 152 by a desired power ratio (e.g.,anywhere from 2:1 to 10:1). The length L2 of phase-shift section 142 isdetermined as described below, in order to achieve the appropriate phaseshift.

In an embodiment, the pitch of grating sections 141, 143 is selected toprovide a λ_(B) equal to the operating wavelength, e.g. 1.31 μm. Forexample, a grating having a period length (pitch) Λ is selected inaccordance with Eq. (1) above, repeated here for convenience:

λ_(B)=2Λn _(e)  (1)

Index n_(e) is approximately equal to n_(G), which is about 3.2333. Toachieve λ_(B)=1.31 μm, where n_(e) 3.2333, use of Eq. (1) results inΛ≈202.6 nm. For a 50/50 duty cycle, each stripe is approximately 101 nmwide. Each stripe is typically 1 to 5 times as high as it is wide. Thus,100 nm wide stripes may be about 100 to 500 nm in height. In anembodiment, stripes 144 are substantially rectangular, having a width ofabout 100 nm and a height of about 150 nm. The “corners” of the gratingprofile may be somewhat rounded, in some applications, e.g. due tofabrication phenomenon such as mass transport.

The effective optical path length Φ introduced in the optical cavity bythe phase-shift section 142 of length L2 is specified in Eq. (2) below:

Φ=L 2·ABS(n _(G) n _(PS))  (2)

In order to achieve the desired phase shift π/2 (i.e., λ_(B)/4) atλ_(B), Φ is set at λ_(B)/4 and Eq. (2) solved for L2 to yield:

L 2=(λ_(B)/4)/ABS(n _(G) n _(PS))  (3)

Where Eq. (3) is true, phase-shift section 142 introduces an effectiveoptical path length of λ_(B)/4, which causes a phase shift of π/2 forlight at λ_(B). For N_(G)=3.2333, n_(PS)=3.23, and λ_(B)=1.31 μm, L2must be about 99.2 μm 100 μm to provide a phase shift of π/2 at λ_(B).Accordingly, in an embodiment, phase-shift section 142 has a length L2of about 100 μm. This section contains no grating but covers a lengthequal to about 500 grating periods of length 202.6 nm.

First and second DBR gratings 141, 143 serve as the mirrors for thelaser cavity and must have sufficient reflectance for lasing to occur,and preferably have a desired asymmetric reflectance ratio, e.g. about2.5:1. The reflectance of a given grating section at λ_(B) is a functionof its length and its grating coupling factor κ. For a grating of lengthL, κL is a key dimensionless parameter determining the frequencyselectivity and performance of a grating, with practical. values of κLtypically being around 2 to 3. The grating coupling factor κ is afunction primarily of factors such as the n, the duty cycle, gratingprofile (i.e. stripe shape), height (thickness) of grating, distancefrom active region 120, degree of optical confinement in the confinementlayers 121, 122, and other factors.

The lengths and other properties of gratings 141, 143 are selected so asto achieve a desired back-to-front reflectance ratio, so that outputbeam 151 is stronger than secondary beam 152, while secondary beam 152is a known fraction of primary output beam 151 so as to be used inmonitoring or other purposes. For example, it may be desired to achievesufficient grating reflectances and a back-to-front reflectance ratio ofabout 2.5:1, and a front-to-back power ratio of about 4:1. In anembodiment, κL=3, L1=65 μm, and L3=135 μm. Thus, in this embodiment, DBRlaser 100 has a total cavity length L L1+L2+L3 65+100+135 μm 300 μm.First or exit grating 141, of length L1=65 μm, contains about 320grating periods (and thus about 320 grating stripes 144); similarly,second or back grating section 143, of length L3=135 μm, contains about670 grating periods. As noted above, phase-shift section contains nograting but covers a length L2 (100 μm) equal to about 500 gratingperiods.

As can be seen, lengths L1 and L3, though different (in an asymmetricembodiment), will still include hundreds of grating periods in order toachieve the desired reflectance, and will be within the same order ofmagnitude. Lengths L1, L3 must include hundreds of grating periodsbecause grating sections 141, 143 are “distributed” reflectors thatnecessarily require many grating periods in order to achieve the desiredreflectance necessary for lasing. For example, L1=65 μm (320 periods) isonly about twice as large as L3=135 μm (670 periods). As will beappreciated, overall cavity reflectivity is lower when the cavitymirrors 141, 143 have asymmetric reflectance, thus increasing thethreshold lasing current. Accordingly, in an embodiment, the gratingcoupling factor K may be increased so as to increase cavity reflectance,to control for the loss in cavity reflectance caused by the asymmetricreflectance.

Length L2, about 100 μm in an embodiment (length of about 500 periods),is also of the same order of magnitude as lengths L1, L3. Length L2 alsohas to cover a length equal to many grating periods (e.g., hundreds),because, as noted above, with stripes of index 3.4 present in claddingof index 3.2 in the grating sections, the effective index N_(G) is onlyslightly greater than n_(PS) (3.2333 is very close to 3.23). Therefore,in accordance with Eq. (3) above, where (N_(G) n_(PS)) is small, acomparatively large length L2, equal to hundreds of grating periods inlength, is needed to achieve the desired λ/4 phase shift. Thus, gratingsections 141, 143, which each include hundreds of grating periods togive rise to the desired reflectance, are distributed reflectors.Analogously, phase-shift section 142 may be referred to as a“distributed” phase-shift section because it extends over a substantialnumber of (missing) grating periods, between the first and secondgrating sections, and is of comparable length to these distributedgrating reflectors 141, 143. In various practical embodiments, none oflengths L1, L2, and L3 is more than about three times the length of theothers.

Simulations were performed by the inventors for a structure similar tothat of DBR laser 100 of FIG. 1, assuming: N_(G)=3.2333, n_(PS)=3.23,λ_(B)=1.31 μm, L1=65 μm, L2=100 μm, and L3=135 μm, κL=3, 0.1%reflectance for AR coatings 101, 102. Simulation of the reflectanceversus wavelength of front and back DBR gratings 141, 143 indicates apeak reflectance for each reflector at approximately λ_(B)=1.31 μm. Inparticular, front grating 141 has a peak reflectance of about 0.32, andback grating 143 has a peak reflectance of about 0.77, at 1.31 μm, i.e.a back-to-front reflectance ratio of about 2.4:1. These reflectances aresufficient to permit lasing to occur in an edge-emitting laser structuresuch as laser structure 100, and also give rise to an asymmetric poweroutput of about 4.2:1, as shown in Table 1, below, which sets forthother simulation results. It is noted that the back-to-front lengthratio L3:L1 is about 2:1, and is related to, but not the identical to,the back-to-front reflectance ratio, because reflectance of a DBRgrating is not a linear function of its length. Additionally, thefront-to-back output power ratio 4.2:1 is related to the back-to-frontreflectance ratio 2.4:1, but is not identical to it.

TABLE 1 Wavelength (μm) factor k back-to-front optical power ratio1.3053682 1.73³ 0.73 1.3061604 1.69³ 0.80 1.3069482 1.32³ 0.85 1.30790421.02³ 2.32 1.3087273 5.88⁴ 0.87 1.3101151 4.11⁴ 0.24 1.3115103 5.71⁴0.85 1.3123437 9.89⁴ 2.32 1.3133057 1.26³ 0.85 1.3141114 1.60³ 0.831.3149188 1.65³ 0.73

Referring now to Table 1, above, column 1 indicates the cavity modewavelength, where the sixth entry (in bold) is the desired operatingwavelength 1.31 μm. Column 2 indicates the imaginary refractive indexfactor κ (the imaginary component of the effective refractive index ofthe cavity) at various cavity mode wavelengths. The imaginary refractiveindex factor k appears in the equation for threshold gain g, which is afunction of wavelength and is given by:

g=k4π/λ  (4)

As will be appreciated, a lower threshold gain g is desired, all thingsbeing equal, and is preferably at a minimum at the operating (Bragg)wavelength λ_(B). A lower magnitude of k results in a lower thresholdgain g, and therefore it is desirable for κ to have a minimum magnitudeat the operating wavelength λ_(B). Column 3 indicates the front-to-backpower ratio (that is, the ratio of the power of main exit beam 151 tosecondary beam 152) provided by the structure of laser 100 at variouscavity mode wavelengths.

As can be seen from the simulation results presented in Table 1, at thedesired operating wavelength 1.31 μm=λ_(B), grating sections 141, 143provide a back-to-front reflectance ratio sufficient to providefront-to-back output power ratio of about 4.2:1 (i.e., the inverse of0.24). Also, it can be seen that the absolute value of the imaginaryrefractive index factor k and thus threshold gain g is approximately ata minimum at the desired operating wavelength 1.31 μm.

During operation, a current is applied via probes, wire bonds or otherterminals electrically connected to metal contacts 133, 111, which pumpsthe active region 120 to cause population inversion and begin the lasingprocess. Light emitted by spontaneous and stimulated emission reflectsback and forth in the longitudinal (y axis) direction in the cavity,along the active region 120, reflected back and forth by DBR gratingsections 141, 142. The light is amplified in the active region, but thetransverse optical cavity extends below and above the active region inthe z direction, i.e. into the confinement layers 121, 122, and alsointo buffer/cladding layer 113, spacer layer 145, and grating layer 140,where Bragg reflection and feedback occurs. Therefore, the light “sees,”in each section 141, 142, 143, an effective index of refraction, whichis due to the combination of the indexes of refraction of the materialof active region 120, as well as material of other layers within thecavity.

As described above, the effective index of refraction N_(G) of gratingsections 141, 143 is slightly greater than the effective index ofrefraction n_(PS) of phase-shift section 142. Given the differencebetween n_(PS) to n_(G), phase-shift section 142 has a length L2 suchthat light traveling through phase-shift section 142 experiences aphase-shift of π/2 at the cavity mode and operating wavelengthλ_(B)=1.31 μm. This phase shift breaks the mode degeneracy that wouldotherwise occur and causes there to be only a single mode with thelowest threshold gain, i.e. the primary mode, at the Bragg wavelengthλ_(B). In other words, without the presence of distributed phase shiftsection 142, the use of grating sections 141, 143 would result in modedegeneracy in which there are two resonance modes having lowest,approximately equal threshold gains. The phase shift section, in effect,“shifts” both of these modes, one (the primary mode) toward λ_(B) (thusdecreasing its threshold gain) and the other, secondary mode, away fromλ_(B) (thus increasing its threshold gain). Because the secondary modehas its threshold gain increased relative to that of the primary mode,the secondary mode and mode degeneracy may be said to have been“suppressed” by the phase shift. There remains only a single, primarymode, at about λ_(B), having the lowest threshold gain.

Thus, use of phase-shift section 142 makes single-longitudinal modeoperation possible, at λ_(B). Laser 100 emits a main exit laser beam 151of wavelength λ_(B) through the facet having AR coating 101 and a backbeam 152 of smaller intensity from its back side, through AR coating102. In single-mode operation, the side-mode suppression is at least aminimum amount, e.g. the a primary mode is at least a certain amount(e.g., 30 dB) greater than any other modes.

Referring now to FIG. 2, there is shown a cross-sectional view of analternative phase-shift section 242 and grating layer 240, in accordancewith an embodiment of the present invention. Grating layer 240 issimilar to grating layer 140 of FIG. 1 except that it containsphase-shift section 242, instead of phase-shift section 142.

As described above, phase-shift section 142 is a section that does notcontain a grating. Therefore, it has a different index of refractionthan the two adjacent grating sections 141, 143, and has the properlength L2 so as to introduce the desired phase shift. Phase-shiftsection 242 also does not contain a grating. Phase-shift section 142 hasno grating by virtue of having no grating stripes 144 or gratingmaterial (e.g. INGaAsP) of which the grating stripes are composed.Alternative phase-shift section 242, by contrast, contains gratingstripe material section 249 but does not contain stripes, since thematerial of grating material section 249 does not contain interveninghalf-periods of material 146, i.e. it is not a grating, it is agratingless slab of material equal in length (L2) to a large number ofgrating periods.

Therefore, like phase-shift section 142, phase-shift section 242 alsohas a different index of refraction than the adjacent grating sections141, 143. However, unlike phase-shift section 142, the effective indexof refraction of phase-shift section 242 is slightly greater than thatof grating sections 141, 143, because it contains on average a greaterdegree of the higher-index grating stripe INGaAsP material (i.e., index3.4) than the lower-index InP material 146 (i.e., index 3.2).Accordingly, the length L2 of alternative phase-shift section 242 isselected in accordance with Eq. (3) above to achieve aquarter-wavelength phase shift.

As described in further detail below with reference to FIG. 3, aphase-shift section 142 having no grating stripes may be formed byremoving, from a continuous grating, the grating stripe material wherethe phase-shift section is to be formed. As described in further detailbelow with reference to FIG. 4, a phase-shift section 242 having agrating stripe material section but with no grating or stripes may beformed by refraining from imparting a grating structure on a gratingstripe material layer, over the region where the phase-shift section isto be formed, when forming the adjacent grating sections 141, 143.

Referring now to FIGS. 3A-H, there are shown cross-sectional views ofthe layer structure of the DBR laser 100 of FIG. 1 at various stages offabrication. These views illustrate a method of fabricating DBR laser100, in accordance with an embodiment of the present invention. First,as shown in FIG. 3A, an initial set of layers are epitaxially grown,e.g. by MBE, onto substrate 112. These include buffer/cladding layer113, bottom confinement layer 121, active region 120, top confinementlayer 122, spacer layer 145, a grating stripe material layer 341, and athin protective layer 342. After the epitaxial growth of layers 113,121, 120, 122, 145, 341, and 342 (in sequence), the sample is removedfrom the MBE chamber for processing steps including depositing a layerof photoresist (PR) 343 on the protective layer 342. PR layer 343 may bethin, about 50 to 100 nm in an embodiment, which is spun onto the wafersurface.

Grating stripe material layer 341 is the material out of which stripes144 are to be formed, e.g a quaternary material such as INGaAsP. Whenthe sample is removed from the MBE chamber vacuum for deposition of PRlayer 343, and during subsequent processing steps, oxide tends to formon the exposed epitaxial layers. Protective layer 342 consists of InPand is used, in an embodiment, because the oxide is easier to removefrom InP than from INGaAsP. In alternative embodiments, protective layer342 is omitted.

Next, as seen in FIG. 3B, the PR layer 343 is patterned to form aregular series of PR stripes which serve as a grating etch mask 345. Thegrating etch mask 345 is formed by any suitable technique. In anembodiment, the grating etch mask is formed by lithographically definingthe grating in the PR layer 343, by exposing the PR layer to twocollimated, expanded beams from a blue or UV laser at an appropriateangle to form high contrast fringes with the desired period. PR layer343 is then developed and the developed portions removed with anappropriate solvent with grating etch mask 345 remaining. Othertechniques for writing a grating into the PR include using electron-beam(E-beam) lithography, a technique which uses special electron-sensitiveresists.

After patterning PR layer 343 to form grating etch mask 345, theunderlying protective layer 342 and grating stripe material layer 341are etched, forming a regular series of grating material stripes 344, asshown in FIG. 3C. At this point in fabrication, each stripe 344 iscovered by a respective stripe from grating etch mask 345 composed ofPR. PR stripes 345 may be removed with any suitable technique to resultin the structure illustrated in FIG. 3D. For example a suitable solvent,oxygen plasma, or other techniques can be used to remove the PR etchmask 345.

Next, as shown in FIG. 3E, a PR mask 351 is formed which covers andprotects the grating stripes which are to form grating sections 141,143, but which leaves exposed the grating stripes that are to be removedbecause they occur in the region where phase-shift section 142 is to beformed. After formation of mask 351, an etch is performed to remove thequaternary material of the exposed grating stripes, resulting in thestructure shown in FIG. 3F. As can be seen, at this point duringfabrication, the sample has two grating sections, separated by anintermediate, gratingless section. Because the two grating sections areformed from an initial, continuous grating, the gratings of the twosections are in phase with each other.

Next, PR mask 351 is removed, e.g with solvent or oxygen plasma, toresult in the sample shown in FIG. 3G. Any oxide may then be removedfrom the sample by appropriate removal or cleaning, and then additionalInP 146 is epitaxially grown to complete grating section 140 andcladding layer 132, as illustrated in FIG. 3H. After the processingillustrated in step 3H, bottom contact 111 may be deposited onto thebottom of substrate 112, and top contact 133 may be formed on claddinglayer 132, including a thin contact-facilitating layer between the metal(Au) of layer 133 and cladding layer 132, to form the laser structure100 shown in FIG. 1.

Referring now to FIGS. 4A-E, there are shown cross-sectional views ofthe layer structure of a DBR laser employing the alternative phase-shiftsection of FIG. 2 at various stages of fabrication. These viewsillustrate a method of fabricating a DBR laser 400, such as a modifiedDBR laser similar to laser 100 except for the different phase-shiftsection, in accordance with an embodiment of the present invention.

FIG. 4A illustrates a stage in fabrication similar to that describedabove with reference to FIG. 3A, except that the sample of laser 400, atthis point in fabrication, contains a thin barrier layer 451 between PRlayer 343 and protective layer 342. This layer extends over the sectionwhich is to contain the phase-shift section 242, and its purpose is toprovide a barrier to prevent the etching of grating stripes in thisregion. Barrier layer 451 consists of a material such as SiO₂, in anembodiment, although other suitable materials, such as or SiO, may alsobe employed in alternative embodiments.

As illustrated in FIG. 4B, a grating etch mask 345 is formed in PR layer343, as described above with reference to FIG. 3B. However, anintermediate portion of grating etch mask 345 covers barrier layer 451,i.e. over the portion to contain phase-shift section 242. Next, aselective etch is performed which is sufficient to etch away the exposedInP protective layer 342 and grating stripe material 341, but which doesnot etch the SiO₂ barrier layer 451. Thus, the resulting structureillustrated in FIG. 4C contains grating stripes in the regions wheregrating sections 141, 143 are to be formed, and a grating stripematerial section 249 (having no grating or stripes) where gratinglessphase-shift section 242 is to be formed.

Next, the PR stripes of PR mask 345 covering gratings 344 of gratingsections 141, 143 and grating material section 249 is removed, asdescribed above with respect to FIGS. 3C-3D, to result in the structureillustrated in FIG. 4D. Any oxide may then be removed from the sample byappropriate removal or cleaning, and then additional InP 146 isepitaxially grown to complete grating section 240 and cladding layer132, as illustrated in FIG. 4E. After the processing illustrated in step4E, as described above with reference to FIG. 3H, bottom contact 111 maybe deposited onto the bottom of substrate 112, and top contact 133 maybe formed on cladding layer 132, including a thin contact-facilitatinglayer between the metal (Au) of layer 133 and cladding layer 132, toform the laser structure 100 shown in FIG. 1.

In conventional DBR lasers, HR (high reflectance) coatings are oftenemployed, instead of AR coatings, on at least one of the laser facets,to provide additional reflectivity for the cavity mirror. One problemwith the use of HR coatings is that the exact place of the cleave isunpredictable. Therefore, the reflectance provided by the HR coatingoccurs at an unpredictable place within the grating period, andtherefore introduces a somewhat unpredictable, random phase shift intoeach laser. This can reduce yield, as only a small percentage of thecleaves occur at the proper location in the grating period so as toachieve the desired mode at wavelength λ_(B) The DBR laser 100 of thepresent invention does not suffer this problem, because only AR coatingsare employed, and the cavity mirrors' reflectance is supplied entirelyby distributed grating sections 141, 143. The facet cleave locationwithin a particular grating period plays almost no role in the phaseshift introduced by the grating, since the grating covers hundreds ofgrating periods. Therefore, use of a grating-only (no HR coating) DBRlaser 100 can be produced with more uniformity of performance (e.g., theoperating wavelength, the front-to-back power ratio, etc.) and greateryield.

Moreover, use of a grating-free phase-shift section 142, which achievesa quarter- wavelength phase shift, can also increase uniformity and thusyield. This is because the gratingless phase-shift section isdistributed, i.e. spread out over the length of many (e.g. 500) gratingperiods. Therefore, slight imprecision in the exact length L2 fromdevice to device causes almost no change in the phase shift effect. Thisis in contrast to gratings having a quarter-wavelength shift inserteddirectly into the grating, in which a very small variation in thisphase-shift section can have a much more significant effect on theamount of phase shift. Further, fabrication of a direct phase shiftsection in a grating requires the fabrication of two or moreout-of-phase gratings, which complicates the fabrication process. Incontrast, the phase-shift section of the present invention may befabricated (as described above with reference to FIGS. 3 and 4) from asingle, continuous grating by removing part of the grating duringfabrication.

Another advantage of employing the phase-shift section 142 of thepresent invention is the achievement of more uniform optical intensityalong the longitudinal (y axis) cavity, which also improves linearity;that is, the range over which a linear increase in output power can beattained is extended. A laser having a directly-insertedquarter-wavelength shift section may lead to an optical intensity“spike” or peak at a given location in the longitudinal cavity,corresponding to the location of the quarter-wavelength section. Thisoptical intensity spike or peak limits the output power that can beachieved without causing spatial hole burning, and thus limits thelinearity of the device. However, because the phase-shift section of thepresent invention is spread out over hundreds of periods and may be ofcomparable length to the two DBR grating sections, there is nocomparable optical intensity “spike” at a single location. Thisadvantageously gives rise to a more uniform distribution of opticalpower over the longitudinal cavity than is achieved by adirectly-inserted phase-shift section which essentially exists at asingle longitudinal location in the cavity.

In addition, the distributed phase-shift section of the presentinvention is not expected to interfere with dynamic wavelength stabilityduring modulation to the degree that a directly-inserted physical λ/4phase shift section does, since it is distributed over a wide lengthequal to many grating periods, instead of occurring at a sharp locationpoint.

As noted above, in an embodiment stripes 144 are substantiallyrectangular in cross-section, i.e. the grating profile (the boundary zbetween the high and low refractive index materials “seen” by lighttraveling in the cavity in the longitudinal, y-axis direction) is asubstantially rectangular periodic wave. In alternative embodiments,other shapes than a rectangular may be employed for the grating profile,such as square, trapezoidal, triangular/sawtooth, or sinusoidal, andduty cycles other than 50/50 may be employed.

In an alternative embodiment, the diffraction grating stripes 144consist of a material having an index of refraction n₁ which is lessthan the index of refraction n₂of the surrounding material 146 ofcladding layer 132 and spacer layer 145.

As described above, in a preferred embodiment L1 L3, so that the lightoutput is not symmetric. In an alternative embodiment, a symmetricdesign may be employed, in which L1=L3, and the reflectances of thefirst and second grating sections are identical, so that the outputbeams 151, 152 have about the same power.

In an alternative embodiment, the grating section having the first andsecond grating sections and the phase-shift section may be disposedbelow the active region (i.e., sandwiched between the active region andthe substrate), instead of above the active region. In anotherembodiment, DBR grating sections are disposed both above and below theactive region, i.e. there are four separate grating sections.

In the preferred embodiments a described above, the gratinglessphase-shift section of the present invention achieves a λ/4 (i.e., π/2)phase shift at λ_(B). In alternative embodiments, length L2 may beselected in accordance with Eq. (2) to achieve a phase shift less thanπ/2 but still significant enough to break mode degeneracy and achievesingle-mode operation. For example, in an embodiment, the gratinglessphase-shift section cause a phase shift by some amount Φ_(PS), whereΦ_(MIN) Φ_(PS) λ_(B)/4, where Φ_(MIN) is the minimum phase shiftsufficient to provide desirable single-mode operation. For example, inan embodiment, Φ_(MIN) is about π/4 (corresponding to λ_(B)/8). Inanother alternative embodiment, Φ_(MIN) is whatever phase shift issufficient to achieve a side-mode suppression of at least 30 dB. Inother alternative embodiments, side mode suppression thresholds otherthan 30 dB may be employed to determine when single-mode operation issufficiently achieved. In still another alternative embodiment, Φ_(MIN)is whatever phase shift is sufficient to cause there to be only one modehaving the lowest threshold gain, which is significantly lower than thethreshold gains of all other modes.

In alternative embodiments, a distributed phase shift section inaccordance with the present invention introduces an odd multiple of λ/4effective optical path length instead of a single λ/4 optical pathlength, to achieve a π/2 phase shift. Thus, the phase shift section may,in general, have a length L2 sufficient to introduce an effectiveoptical path length of (2n+1)λ/4, where n is an integer {0, 1, 2, 3,...}, which achives a π/2 phase shift. However, n=0 is employed in thepreferred embodiments described above because a longer phase-shiftsection reduces the available length for reflective optical feedback inthe grating sections or, conversely, requires a longer laser cavity inorder to accommodate the longer phase-shift section.

In other embodiments, materials, material systems, and layer thicknessesfor the layers or elements of laser 100 other than those specifiedherein may be employed. In alternative embodiments, a laser structurehaving first and second DBR grating sections and a gratingless phaseshift section in accordance with the present invention may have lessthan all of the layers and elements shown in the embodiments describedabove, and may have additional layers or structures not shown. Forexample, in an alternative embodiment, there are no optical confinementlayers 121, 122, and/or no spacer layer 145, and/or no thin protectivelayer 342 employed during fabrication of grating layer 142 of structure100. And, for example, in an alternative embodiment, an etch stop layercomposed of a quaternary material such as INGaAsP may be disposed in thecladding layer 132, between its boundary with top contact layer 133 andgrating stripes 144.

In the present application, a “non-section-112(6) means” for performinga specified function. is not intended to be a means under 35 U.S.C.section 112, paragraph 6, and refers to any means that performs thefunction. Such a non-section-112(6) means is in contrast to a “meansfor” element under 35 U.S.C. section 112, paragraph 6 (i.e., a“section-112(6) means”), which literally covers only the correspondingstructure, material, or acts described in the specification andequivalents thereof.

The present invention, therefore, is well adapted to carry out theobjects and attain the ends and advantages mentioned, as well as othersinherent therein. While the invention has been depicted and describedand is defined by reference to particular preferred embodiments of theinvention, such references do not imply a limitation on the invention,and no such limitation is to be inferred. The invention is capable ofconsiderable modification, alteration and equivalents in form andfunction, as will occur to those ordinarily skilled in the pertinentarts. The depicted and described preferred embodiments of the inventionare exemplary only and are not exhaustive of the scope of the invention.Consequently,.the invention is intended to be limited only by the spiritand scope of the appended claims (if any), giving full cognizance toequivalents in all respects.

What is claimed is:
 1. A method for fabricating a laser for generatingsingle-longitudinal mode laser light at a lasing wavelength, the methodcomprising the steps of: (a) forming a semiconductor active region foramplifying, by stimulated emission, light in the laser cavity at thelasing wavelength; (b) forming a grating adjacent to the active region,the grating having a grating period corresponding to a Bragg wavelengthsubstantially equal to the lasing wavelength; (c) removing anintermediate section of the grating to result in first and secondpluralities of gratings separated by a gratingless intermediate section;(d) forming a first grating section comprising the first plurality ofgratings and having a first reflectance and a first effective index ofrefraction; (e) forming a second grating section comprising the secondplurality of gratings and having a second reflectance and the firsteffective index of refraction, wherein said second reflectance isgreater than said first reflectance; and (f) forming a gratinglessphase-shift section in said intermediate section, the phase-shiftsection being disposed adjacent to the active region and between thefirst and second grating sections and having a second index ofrefraction different than the first effective index of refraction,wherein the phase-shift section has a length sufficient to impart aphase shift for light at the lasing wavelength sufficient to achievesingle-longitudinal mode operation and each of the lengths of the firstgrating section, the second grating section, and the phase-shift sectionis no more than about three times the length of the other two of saidsections.
 2. The method of claim 1, wherein the first and second gratingsections are distributed Bragg reflectors.
 3. The method of claim 1,wherein the first and second grating sections have a grating periodselected so that the Bragg wavelength of the grating sections issubstantially equal to the lasing wavelength.
 4. The method of claim 1,wherein the phase-shift section is a distributed phase-shift section. 5.The method of claim 1, wherein the phase-shift section imparts a phaseshift of about π/2 for light at the lasing wavelength.
 6. The method ofclaim 1, wherein the phase-shift section imparts a phase shift of atleast about π/4 for light at the lasing wavelength.
 7. The method ofclaim 1, wherein the phase-shift section imparts a phase shift for lightat the lasing wavelength sufficient to achieve single-longitudinal modeoperation having at least 30 db side-mode suppression.
 8. The method ofclaim 1, wherein the first and second grating sections each comprise aplurality of grating stripes composed of a grating stripe materialsurrounded by a cladding material, wherein the phase-shift sectioncomprises the cladding material but no grating stripes.
 9. The method ofclaim 8, wherein the grating stripe material consists of INGaAsP and thecladding material consists of InP.
 10. The method of claim 1, whereinthe first and second grating sections each comprise a plurality ofgrating stripes composed of a grating stripe material surrounded by acladding material, wherein the phase-shift section comprises agratingless section of grating stripe material surrounded by thecladding material.
 11. The method of claim 1, wherein the lasingwavelength is about 1.31 μm.
 12. The method of claim 1, wherein thelasing wavelength is about 1.55 μm.
 13. The method of claim 1, furthercomprising the step of depositing a first anti-reflectance (AR) coatingon a facet adjacent to and perpendicular to the first grating sectionand depositing a second anti-reflectance (AR) coating on a facetadjacent to and perpendicular to the second grating section.
 14. Themethod of claim 1, wherein the active region comprises a plurality ofquantum well structures.
 15. The method of claim 1, wherein the activeregion comprises a semiconductor material adapted to amplify, bystimulated emission, light in the laser cavity at the lasing wavelengthat around the lasing wavelength.
 16. The method of claim 1, wherein thefirst and second grating sections and the phase-shift section areparallel to the active region.
 17. The method of claim 1, wherein thesecond index of refraction is greater than the first effective index ofrefraction.
 18. The method of claim 10, wherein the second index ofrefraction is greater than the first effective index of refraction. 19.A method for fabricating a laser for generating single-longitudinal modelaser light at a lasing wavelength, the method comprising the steps of:(a) forming a semiconductor active region for amplifying, by stimulatedemission, light in the laser cavity at the lasing wavelength; (b)depositing a grating material layer adjacent to the active region, thegrating material layer having a first section, a second section, and anintermediate section between the first and second sections; (c) forminggratings in the first and second sections only to form a first gratingsection in the first section, a second grating section in the secondsection, and a gratingless phase-shift section in said intermediatesection, the first and second grating sections having a grating periodcorresponding to a Bragg wavelength substantially equal to the lasingwavelength, the first grating section having a first reflectance and afirst effective index of refraction, the second grating section adjacentto the active region and having a second reflectance and the firsteffective index of refraction, and the phase-shift section having asecond index of refraction. different than the first effective index ofrefraction, wherein said second reflectance is greater than said firstreflectance, the phase-shift section has a length sufficient to impart aphase shift for light at the lasing wavelength sufficient to achievesingle-longitudinal mode operation, and each of the lengths of the firstgrating section, the second grating section, and the phase-shift sectionis no more than about three times thereof of the other two of saidsections.
 20. The method of claim 19, wherein the second index ofrefraction is greater than the fist effective index of refraction. 21.The method of claim 20, wherein the first and second grating sectionseach comprise a plurality of grating stripes composed of a gratingstripe material surrounded by a cladding material, wherein thephase-shift section comprises a gratingless section of grating stripematerial surrounded by the cladding material.